This invention relates to imaging sonar systems of the type used on boats and ships in detecting and displaying underwater targets as the bottom and the shore, and mobile objects such as fishes.
Heretofore, imaging sonar systems have employed transducer means for generating sound waves and for detecting echoes therefrom. Typically, the transducer is continuously rotated so that the body of water about the system is scanned omnidirectionally. Detected echoes are fed as an analog video signal to a cathode ray tube where they are displayed along a rotating sweep line synchronized with the rotating transducer. Through usage of long persistence phosphor the image displayed on the tube trails the sweep line a sufficient length of time to enable the operator to mentally visualize a sizable portion of the area under surveillance at any one time.
The just described imaging sonar systems of the prior art have met with only limited commercial success due to a number of performance limitations. A principal problem found by operators using such imaging sonar systems has been the degree of difficulty in interpreting the displayed image. Due to the relatively slow velocity of sound in water the transducer must rotate at a relatively slow rate. As a result, the sweep on the cathode ray tube accompanying rotation of the transducer is itself slow. Thus, even with the use of long persistence phosphorous tubes, the majority of the display is still blank at any one moment. The operator therefore is forced continuously to remember images previously presented in order to form a mental picture of the entire area being surveyed. With a boat moving over the bottom and with fishes themselves swimming therebeneath, this places substantial mental stress upon the operator.
In addition to the just mentioned limitation of imaging sonar systems of the prior art, discrimination between echo signal strengths has also been exceedingly difficult to obtain since the signal received by the cathode ray tube is analog. For example, weed-covered and muddy bottoms appear as a relatively weak echo return. To insure that the bottom is presented to the operator, it is necessary to have even its weak signals energize the tube phosphor. Thus, when relatively strong echoes, such as those caused by rocks, appear, they have a strength of return several magnitudes of order greater than that of a muddy bottom. As a result, a host of returns falls into the category of maximum phosphorous illuminescence rendering indiscriminate patterns of echo return on the display tube. The absolute intensity of long persistence phosphor is also relatively low requiring the tube to be light shielded or placed in a darkened room which further lessens its attractiveness to small boat users such as fishermen.
In addition to operational deficiencies, prior art imaging sonar systems have been technically complex and expensive to manufacture. Cathode ray tubes of the type that employ relatively long persistence phosphor are quite costly. High azimuth resolution has been required in view of the above mentioned difficulties in target discrimination. In order to completely scan the body of water beneath a boat it has been necessary not only to have scan horizontally in azimuth but also in elevation. This in turn increased the complexity and cost of prior systems. Furthermore, it has been necessary to synchronize the movement of the transducer in the water with the sweep of the cathode ray tube. This is inherently difficult to accomplish with simplicity due to the many mechanical forces to which a transducer submerged in water beneath a moving boat is subjected. To exclude extraneous electric noises, such as those of spark plugs of the boat motor, the prior art devices have also had to use mechanically tuned filters which themselves present added cost. A further disadvantage has been the incapacity of the prior art systems to recall prior images.
Accordingly, it is a general object of the present invention to provide an improved imaging sonar system.
Another general object of the invention is to provide an improved method of forming visual images of submerged scenes.
More specifically, it is an object of the present invention to provide an imaging sonar system having improved image display capacity.
Another object of the invention is to provide an imaging sonar system utilizing a cathode ray tube of the type employing short persistence, high intensity phosphors.
Another object of the invention is to provide an imaging sonar system and method by which a complete and substantial current image of an area being scanned may be continuously presented for observation.
Another object of the invention is to provide an imaging sonar system capable of displaying echo returns with improved target strength discrimination.
Another object of the invention is to provide an imaging sonar system employing a fixed elevation angle transducer.
Another object of the invention is to provide an imaging sonar system which does not require synchronization between transducer position and cathode ray tube display sweep.
Another object of the invention is to provide an imaging sonar system with long term image storage means for recalling previously displayed images.
Another object of the invention is to provide an imaging sonar system capable of displaying images on a cathode ray tube without substantial fading of images.
Yet another object of the invention is to provide an imaging sonar system with improved means for rejecting external noise and for synchronizing operating frequencies of the transmitter and receiver.
Still another object of the invention is to provide an imaging sonar system of the type described which is of relatively simple, reliable and economic construction.
Still yet another object of the invention is to provide a method of forming visual images of submerged scenes by which data in the form of sound wave echoes is received at a substantially slower rate than that by which data is presented in digited electrical form to a cathode ray tube for display.